Ozone bleaching is gaining more and more acceptance in the pulp and paper industry and is now commercially employed in a pulp mill in Austria to treat a sulphite pulp.
It is known that pretreating pulps containing cellulose and lignin with nitrogen oxides prior to oxygen bleaching improves the bleaching operation. This concept is discussed in a paper by Dennis Rasmussen and Olof Samuelson entitled Nitrogen (II) and Nitrogen (III) Compounds During Treatment of Kraft Pulp and Kraft Lignin with NO.sub.2 published in Nordic Pulp and Paper Research Journal No. 3, 1988, pages 138-144.
A number of different authors have described the use of nitric oxide which converts to nitrogen dioxide (NO.sub.2) on contact with oxygen (O.sub.2) as a pretreatment prior to ozone (O.sub.3) bleaching. These teachings discuss treating the pulp with NO.sub.2 at temperatures in the order of about 70.degree.-90.degree. C. and the results showed no increase in the ratio of viscosity to permanganate number over the control. Attention is directed specifically to the following publications.
1. Optimization of Bleaching Sequences using Peroxide as a First Stage, 1982 International Pulp Bleaching Conference, B. Lachenal et al.
2. Application of Nitrogen Dioxide and Ozone in the Pre-Bleaching of Kraft and Sulphite Pulps, 1985 International Pulp Bleaching Conference, Pratt et al.
3. Improvement of Ozone Pre-Bleaching of Kraft Pulps, International Pulp Bleaching Conference, 1985, Latchenal et al (see also Journal of Pulp and Paper Science, Volume 12, No. 2, March 1986).
4. Bleaching of Bagasse Soda Pulp with Nitrogen Dioxide and Ozone, Holzforzchung, 39 (1985), pp.305-310 by Pratt et al.
The paper Laboratory and Pilot Plant Bleaching of Various Pulps with Ozone by Pratt et al, 1984 Oxygen Delignification, pp.33-42, describes experiments carried out by Pratt et al wherein pulp was treated with NO.sub.2 gas and ozone. Generally the treatment occurred in a refiner and in one example the treatment was carried out at a consistency of 40% at a temperature of 30.degree. C. (the paper indicates on page 36 that the optimum temperature for nitrogen dioxide treatment is around 70.degree. C. however because they were applying a combined NO.sub.2 /O.sub.3 treatment an intermediate temperature to the optimum temperatures for ozone and NO.sub.2 was applied). The conclusion was that for those treatments incorporating NO.sub.2, the yield, bleachability and mechanical pulp properties were inferior to those obtained with pure ozone stage.
It is suspected that the amount of NO.sub.2 added which was always maintained low (about 1% on the oven dry weight of the pulp) produced no benefit.